A problem in automatic drawing and in the use of automatic drawing apparatus is to maintain a constant, sufficiently rapid flow of the writing fluid, despite a high drawing speed to draw uniformly wide and fully covering lines. On the other hand, it must be ensured in automatic drawing that no ink will flow or begin to drop after a line is completed, that is, after the tubular pen has been lifted, and that particularly no drop will form on the tubular pen since this could lead to inkspots when the pen is placed again on the drawing surface.
A drawing instrument is already known (DOS No. 2,235,737), where it has been tried to solve the above mentioned problems by regulating the volume of the writing fluid in the tubular pen, whereby the amount of writing fluid to be supplied during drawing is controlled by means of a speed signal generator connected to the drawing instrument to generate a speed signal which indicates the relative movement between the tubular pen and the drawing surface. A pump is controlled corresponding to the speed signal in order to supply writing fluid to the tubular pen, thus ensuring at all times a continuous adequate supply of writing fluid during drawing.
It is a disadvantageous fact that this type of known drawing instrument has considerable inertia, inherent in its complicated mechanical design, so the supply of writing fluid cannot be stopped without delay therein when the drawing process is completed. Therefore, when the tubular pen is lifted from the drawing surface the entire assembly carrying the tubular pen continues to move on so that a speed signal is generated and some writing fluid is additionally supplied to the tubular pen. This additional fluid supply results in the formation of an ink drop on the writing tubule, so that either inkspots are formed or lines are widened when the pen is applied again.